The Fallon Co. broke ground today on the 500,000-square-foot One Fan Pier Boulevard, which will be the first office structure to grace the high-end 3 million-square-foot fan Pier mixed-use development in Boston. Spanning 21 acres along the harbor, the entire development – already home to the one-year-old Institute of Contemporary Art-will cost$3 billion to complete and will be the largest green project in the city.